


Made to Kill

by LysandraKenway



Category: Dr. STONE (Anime), Dr. STONE (Manga)
Genre: Aged-Up Character(s), Alternate Universe, Assassin Kohaku, Enemies to Friends to Lovers, F/M, Slow Burn
Language: English
Status: Completed
Published: 2020-01-16
Updated: 2020-01-18
Packaged: 2021-02-27 11:53:39
Rating: General Audiences
Warnings: No Archive Warnings Apply
Chapters: 4
Words: 7,871
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/22226695
Author URL: https://archiveofourown.org/users/LysandraKenway/pseuds/LysandraKenway
Summary: Kohaku, a world-class assassin, is hired to kill Ishigami Senku, but things don't go according to plan.
Relationships: Ishigami Senkuu & Kohaku, Ishigami Senkuu/Kohaku
Comments: 38
Kudos: 119





	1. Chapter 1

**Author's Note:**

> The lack of SenkuxKohaku art/fics is making me feel like I'm the only one in the world who ships these two. I hope I'm wrong, though. If you're a fellow shipper, I hope you enjoy reading this, and leave me your feedback in the comments! 
> 
> P.S: The relationship is a bit slow to develop so... sorry about that.

Kohaku checked her watch and yawned. It wasn't her first time attending a fancy gathering, but that didn't make her hate them any less. She dreaded formal attire, especially since her bodycon dress was constricting her mobility and barely doing a decent job at hiding her weapons—which she would have liked to bring more of, but only managed to sneak a combat knife and a switchblade. Well, all that mattered was that she had successfully infiltrated the event that was held in the honour of the scientist she was contracted to kill. The only problem was that she couldn't locate him anywhere. 

As the evening came to a close, it was finally announced that the scientist was not going to make it, so the the event was cancelled and the award he was to receive was going to be personally delivered to his residence. 

That, however, sounded extremely suspicious to Kohaku. Throughout the years, this elusive scientist had kept such a low profile that no one, in fact, knew where he lived, not the most skilled hackers, not the best assassins, and not even the government—hence her targetting him at an event as opposed to finding his house—so this begged the question: how was the award going to be delivered to an unknown location? The way Kohaku saw it, there was only one way to find out. 

* * *

After two sleepless nights of intensive tracking, Kohaku managed to find the scientist's residence, and finally understood why no one knew where lived.

His house was located on an uncharted island that could only be accessed by a remote-controlled jet, a ridiculously over-the-top security measure, some would say, but not Kohaku, for she had managed to sneak onto the plane with no difficulty, and even made her way to the scientist's doorstep with absolute ease. All she had to do was follow a robot that came to pick up the prize. There were no cameras, no motion sensors, nothing at all, but once she thought about it for a moment, she realized that her task had been a little _too_ easy, disquietingly so, in fact. And for that reason, she started to have a bad feeling about her mission.

As the robot entered a large futuristic-looking building which she presumed to be the scientist's house, she stealthily slipped inside right behind it, guns aimed ahead of her, but just before she could make her next move, the robot spoke.

“Senku, I brought your prize from the jet, and the guest you were expecting has followed me, as you said.”

Kohaku swallowed a lump that was starting to form in her throat. Was she the guest the robot was talking about? Had it been aware of her presence the whole time? 

“Put the award with the rest, and bring something for our guest to drink," said a voice coming from a distant room. "She must be tired.” 

She was undoubtedly the guest that the robot was referring to. If the scientist knew her gender then he definitely did have cameras, and if he was able to see her, then he definitely knew she was armed. This scientist was ready for her, but why hasn’t he taken action yet? How and more importantly _why_ was she still alive?

“Well, then, Miss Assassin,” started the scientist, strolling out of a room, and taking off the goggles and gloves he had on without even so much as taking a glance her way. “Welcome to my humble abode. I hope you no longer have any intention of killing me because I'm planning to share my newest project with the rest of the world today,” he added as he examined his new award.

Why wasn’t she shooting him? Why did she freeze?

‘Shoot him, Kohaku!’ she screamed at herself internally. ‘There’s nothing standing in your way! Why aren’t you shooting, you idiot?!’

The answer was simple: he knew she was coming to kill him, but he hasn’t killed her, and she wanted to know why. What was he playing at? Why did he stroll out of his room so confidently without paying her any attention? Was he wearing a bullet-proof vest? No, his head was exposed. Were there guns pointed at her from somewhere she couldn't see? It wasn't a far-fetched possibility. If someone with 11.0/10 vision like her couldn't spot any cameras anywhere despite them being there, then she could expect all sorts of surprises.

“I know what you’re thinking,” he said, interrupting her thought process, and then finally turning to meet her gaze. "'Why hasn’t he killed me if he knows I’m here to kill him', right? 'He must be planning something'. Well, to tell you the truth, I’m really not planning anything, but that doesn’t mean I’m just planning to die either," he continued in a more serious tone. "Now, tell me, assassin. Why were you hired to kill me?”

But just before she could open her mouth, Kohaku sensed movement to her right, and in her split second of panic, she reflexively fired in that direction. It turned out that what was approaching her was the robot the scientist had ordered to bring her a drink, and indeed, it had a plate with three drinks on it. 

“Wat-errrrrr, Juice or W-ine?” said the robot in a distorted accent that was probably due to the several bullet holes in its head.

She glanced at the scientist in suspicion, but he was quick to abate her concerns.

“The drinks aren’t poisoned, but feel free not to drink any if you want to play it safe,” he said crossing his arms behind his head, as he sat on a nearby sofa. “Now, then. When are you planning to answer my question? I don’t have all d—”

“Why haven’t you killed me yet?” she quickly interrupted.

“I don’t want to,” he shrugged. “I barely get any visitors. Plus, you’re just in time for my latest invention’s testing. Come on," he said, as he got up. "I’ll show it to you.”

“Don’t move or I’ll shoot!” she forewarned, only to have him completely ignore her and continue walking towards a certain room.

“It’s an electro-magnetic-field generator that neutralizes any outside forces. I'm 10 billion percent sure someone like you would love it,” he bragged. 

The scientist soon vanished into a room leaving a hyperventilating Kohaku frozen in place. The harder she tried to wrap her head around his strange behaviour, the more bewildered she grew. The only possiblity that remotely made sense to her was that he was a manipulative maniac who had willingly allowed her on his island so he could have fun testing his weapons on a live subject.

What if she followed him into the room and he blasted some lazer weapon in her face? Kohaku had inadvertently put herself in a bad situation, that she could see no way out of. No matter what she did, she was certain that this scientist was going to be one, if not several, steps ahead of her. 

“You coming or not?” said the scientist, sticking his head out of the door frame. “I thought you were here to kill me. You’re not doing a great job at it, you know,” he added mockingly as he disappeared back into the room. 

The scientist's cold remark struck Kohaku like a bolt of lightning. She was supposed to be one of the top assassins of the world, known for her quick-thinking skills and extreme agility, but she was definitely not acting like it.

Overridden with shame, she finally pulled herself together, and with her focus regained, carefully made her way towards the room.

“Finally! I was starting to think you weren't going to make it," he said sarcastically. "Now, why don't you go ahead and try to shoot me?" he challenged. 

Kohaku was perplexed. First this person shows no signs of alarm at seeing an assassin in his house, going as far as even referring to her as a guest, and then he asks her to shoot him... Either she was somehow missing the bigger picture, or something was clearly wrong with the man. 

“Go on. Don't just stand there all day, or do you need me to remind you why you're he—”

Convinced that she was going to die no matter what happened, Kohaku closed her eyes and fired her guns at the scientist.

"You can open your eyes now," he said, causinge Kohaku's eyes to shoot open in dibelief as she registered the fact that both of them were still somehow in one piece. "I’m perfectly fine! Isn’t that crazy?!”

It was impossible... Did she miss?!

“Those were some pretty good shots, might I add. One between the eyes and one straight for the heart. Can’t expect any less from a professional gun-for-hire," he stated. "Come on, do it again, and keep your eyes open this ti—”

Kohaku didn't allow him to finish his sentence and, eaten by fury, started shooting again, moving closer with each bullet, but only to have every one of them fall to the ground as soon as they reached him.

If it was science that was stopping her gunfire, then it was science on a level she'd never seen before. If anything, it was bordering on science-fiction. Whatever it was that made her guns seem completely useless—because she really couldn't _see_ anything around the scientist—she didn't want to admit defeat to it, so even after her guns ran out of bullets, she refused to stop. Instead, she pulled out her knives, and started going for the scientist, but to no avail. 

“You can stop now,” he sighed. “I think you’ve established that you’re not going to get through the force field.”

She coulnd't stop, though. Nor did she want to. She wasn't going to allow the scientist to relish in humiliating her.

"Alright, that's it. I'm taking it down," he declared in exasperation.

As soon as he said he was taking down the force field, Kohaku's knives actually reached the scientist's body, forcing her to bring her momentum to a sudden halt almost a bit too late.

Trickles of blood ran down the scientist's neck and his left arm, where the knives had made contact with his skin, and as soon as he lifted a hand to wipe the blood on his neck, Kohaku pinned him to a wall.

“Why did you drop the force field?" she snarled. "What makes you think I’m not going to kill you?” 

“Are you going to?” he asked slyly.

“Just answer the damn question,” she growled, pressing the knife further against his throat.

“If you wanted to kill me, you would have done it by now, but there’s something that’s stopping you.”

Looking fixedly into the scientist's ruby eyes enfuriated Kohaku. They were calm. His whole face was just serene, as if one of the planet's most skilled assassins posed no threat to him whatsoever. But she knew that that was not the case. She could feel his arm trembling in her grip. He _was_ scared. It was just that was certain that she wouldn’t take his life, and his confidence in his own conjecture made Kohaku's blood boil. He was right, but how was he so sure of it? What was this man? Was he even human? Maybe that was the reason she was told he was a menace to humanity and that his scientific prowess was going to bring the world to its demise. That was why she was hired to kill him, after all, but however ironic it might have been, that was also the very reason she couldn't bring herself to do it.

This scientist... Senku Ishigami, rekindled some flames of hope inside her that had been slowly withering and dying for almost 7 years. If this man could singlehandedly build a machine that defied human comprehension, and accurately predict the actions of someone he knew nothing about, then maybe—just maybe—he could put an end to the bane of her existence.

“I’ll make a deal with you," she said assertively. "In exchange for you life, you're going to help me.”

“Sounds fair enough, but I need to know what is it that you want help with first.”

Was he being serious? How could someone be discussing conditions of agreement when their life was on the line? It baffled Kohaku, but at least it gave her the opportunity to ask a very important question that she should have asked before proposing the deal in the first place.

“How much do you know about medicine?”

“Hmm. You look awfully healthy to me, so I’m guessing someone important to you suffers from an incurable disease, is that it?”

Damn it. How could this man jump from one right conclusion to the other so effortlessly? 

“It’s… It’s not incurable," she stammered, saddened by the thought of her bed-ridden sister. "It’s just…” 

“Tell you what; talking with a knife at my throat is pretty uncomfortable, so how about you put it down so we can talk like civil adults?”

“Shut up!" she yelled, snapping out of her brooding state. "You just want me to put it down so you could kill me!”

“I told you that I don’t want to kill you already, you idiot,” he said in a frustrated tone. “And assuming I did want to kill you, why would I have dropped the electromagnetic field and put myself in this situation to begin with, hein?”

As much as Kohaku would have hated to admit it, he had a point. It was clear that he had no intention of killing her, and considering the circumstances, it was about time she gave up on trying to kill him as well.


	2. Chapter 2

**Notes for the Chapter:**

> (POSSIBLE) SPOILER ALERT: The following chapter(s) may contain spoilers for two characters yet to be introduced in the anime (specifically, their names and some vague character traits, but nothing plot-related. You may consider the two characters as my OCs and not really think of how they would feature into the orginal work.)

Convinced that there was no longer any point in her empty death threats, Kohaku slowly loosened her grip over the scientist, and took a few steps back.

“So...tell me about this illness,” he said.

And Kohaku obliged. She told him eveything about her sister Ruri’s condition; about how doctors had never seen anything like it before, nor knew how to treat it; about how studying her case and developing a cure would require months or possibly years of experimentation and about how her sister most likely didn't have that much time left.

“So is that why you became a gun-for-hire?" he asked, in a seemingly sympathetic tone that took Kohaku off-guard. "To support your sister's medical costs?” 

“Yeah,” she admitted bitterly. 

“Really?" he sneered, instantly ruining the good impression that Kohaku was starting to form of him. "Was that the only lucrative job you could think of? Did you drop out of high-school or something?”

“Who are you calling a drop-out, you…?!” she flared, taken aback by his rudeness. “I didn’t drop out," she grumbled as she crossed her arms. "I was about to start going to college when Ruri’s state started getting worse. My dad’s job wasn’t enough to cover the medical bills so … I made a choice.”

But Kohaku decided to leave out an important part of her story: the fact that she wasn't any kind of assassin. She had a code. She only killed people who posed a threat to humanity: corrupt people, criminals, terrorists etc. But the only reason she decided to leave that part out was because she had never really thought of herself as some righteous vigilante. All she cared about was saving her sister, so whether society viewed her as some monstrous threat or as a valiant hero mattered little to her, and she might have been crazy for thinking it, but she had the impression that it mattered little to the scientist as well. 

“So back to my very first question. Why does your current client want me dead, again?” he asked as he pulled out some machine and started tinkering with it.

“Dangerous... Might start World War III... Creates nukes and weapons of mass destruction. The list goes on.”

“Sounds illogical. If the person who sent you had the good of mankind in mind, they wouldn't have built their claims on rumors and would have taken into consideration my contributions to ending world hunger, limiting pollution and solving energy resource problems. In other words, the person who sent you has another more personal motive, and I'm 10 billion percent sure it was that bastard Ibara.”

Kohaku had no reply. She was simply in awe of the scientist's deductive skills. 

"So it _was_ him..." he started when Kohaku didn't deny any of his claims. "Which means my theory is correct: you don't kill people indiscriminately. You only kill specific kinds of people, don't you?" 

"How did you know that?" asked Kohaku, arching her eyebrows in confusion.

"While you were on your way here, I had a look at your previous targets and noticed a pattern," he said. "That would explain why Ibara lied to you to get you to do the job, but there is still something that I don't understand. If you only kill 'bad people' then you surely cross reference your clients' claims, right?"

"All the sources I found said that you were dangerous," she objected, having understood that he was accusing her of negligence. "It wasn't just Ibara." 

"Then you didn't do enough research," he shrewdly replied. "Say, how much is Ibara paying you?" 

Kohaku pouted, and looked away from the scientist. He was clearly implying that the money blinded her from doing enough research on him, but that wasn't the case. Not entirely at least...

It was true that the sum Ibara offered was large, but she only accepted it because it would be more than enough to cover years' worth of her sister's medical expenses, if not altogether allow her to buy an entire research facility that she would dedicate solely to the study of Ruri's disease. So the fact of the matter was that she didn't really have much of a choice other than to half-heartedly accept the job. She did feel guilty and slightly ashamed of herself, but she eventually ended up not killing the scientist, so all's well that ends well.

"That much, eh?" he finally said when Kohaku didn't reply. "Heh heh heh, guess that crazy bastard won't stop until I'm dead," he snickered. 

"Why does this guy want you dead so badly anyway?" 

"Let's see... long story short, he bet his success on me selling him one of my inventions, but since I _don't_ sell my inventions, his life was ruined, so instead of blaming himself for being an illogical dingus, he blamed me and made it his goal to ruin my life as payback... until he realized that wasn't going to work, which is when he just decided to start sending hitmen after me."

"Hitmen? You mean he's sent people after you before?" 

"That is correct."

"Then how is he not in jail?"

"Heh heh heh, good question. Looks like you're not all brawn, after all," he said sardonically. "Well, after our fallout, he somehow managed to rise to power and make a lot of money, and, more importantly, connections: judges, law-enforcers, etc... You get the picture."

With all her questions answered, Kohaku deemed the scientist's story believable and decided that he wasn't the manipulative maniac she was expecting him to be. As such, she found herself curious to know more about him—independently of his backstory with Ibara, that is—which lead to her to asking a strange question. 

“And…what happened to the other people that Ibara sent to kill you?”

“They’re in prison, obviously?" he replied sarcastically. "What, did you think I killed ‘em or something?” 

That was, in fact, not what Kohaku thought. The reason she asked was much more embarassing: she wanted to know whether Senku's preferred method for avoiding death was having a heart-to-heart with all his assassins or whether she was the sole the exception. 

“No, it's just... uh," she stammered as her face colored. "How would I know?!" she finally blurted.

"I don't kill people," he said in a grave tone. "With science, there's always a way to defend yourself without hurting anyone.”

"Like that device you showed me earlier..." she mused.

"Heh heh heh, so you _did_ think it was cool?"

"Well of course I did! I mean, anybody would. It’s not like it’s something one gets to see every day..." she dismissed, attempting to downplay her fascination with his work. "Anyway... are you going to help my sister?”

"Hmm, I _was_ planning to go to Mars next week...but I guess I can put it off for a little longer,” he said nonchalantly.

"Did you just say you were going to Mars?" she exclaimed in disbelief.

"Why do you sound so surprised? Aren't you supposed to know I work with NASA?" 

"I-I do... It's just that humans haven't been to Mars before, so it's kind of a big deal. I thought NASA would make an announcement or something..." 

"Oh they're not going yet. They have no reason to be making an announcement so soon. I'm going on my own accord." 

Why did everything he say sound so puzzling? Was he trying to make her head spin on purpose?

"Uh... what?" 

"Heh heh heh. I think it'll be better if I just showed you," he said with a mischievous grin. "Follow me." 

Senku then took Kohaku to a vast underground space in which she saw something she would have never imagined any one individual on earth could own: a rocket ship! Yes, Senku had his very own spaceship… The fact that he worked with NASA clearly wasn’t enough for this guy. He had to go and make his own ticket to outer space so he could leave the planet whenever he pleased.

“So you... you just... go to space? Like... whenever you want?” she stumbled over her words.

“Don’t be illogical. That would be a waste of fuel.”

"Right... And you said you were planning to go to Mars on your own next week?" 

"After I finish the new modifications to the engine, yes." 

“But does NASA know about this? I mean, ar-are you even _allowed_ to do that?”

"Heh heh heh, they were the ones that challenged me to do it in the first place. It was a dumb bet," he explained. "Whoever manages to build a spacecraft that could reach Mars in less than 3 months by next week gets to name the next galaxy after them.”

The fact that Senku referred to a potentially life-altering event as a "dumb bet" baffled Kohaku. She could clearly see it, now. This man named Senku was insane. He was a mad scientist par excellence, but she would have been lying if she said that she found him to be anything short of amazing.

“Ishigami Senku,” she started. “If you cure my sister, I promise I’ll kill Ibara and anyone else who would even dream of standing in your way.”

“Heh heh heh, why so dramatic? There’s no need for you to kill anyone. I have science to protect me. Besides, we have to cure your sister first. There’s always a chance that I might fail, but in order to know for sure we need to get her here so I could have a look at her.”

Kohaku bit her lip and a somber expression slowly invaded her features.

“But… she can barely walk two steps outside of her hospital room. What if she doesn’t survive the trip?" she started then quickly changed her mind. "No. You’re right. If there's even the slightest chan—”

“Negative," he quickly cut in. "If she can’t move, then you have to bring me her three samples: blood, saliva, and urine. We might need other samples later on too, and if that turns out to be the case, you’re going to be in for a lot of delivery service. Can you handle that, lioness?”

“Yes," she replied instantly, eyes shining with determination. 

“Well, I’m not sure you understood me correctly," he clarified. "What I meant is that if you agree to do this, you wouldn't have enough time to take on other job offers. Is that going to be ok?”

Not only was this man willing to help her cure her sister, but he was considerate enough to make sure he wasn't putting either of them through even the slightest of inconveniences, and realizing that made a melange of emotions mingle in Kohaku's chest.

"I have enough money at the moment," she replied. "But if it runs out, I'll figure out a way to manage.”

"Alright. There's no time to waste then. You can go back to your sister and bring me the samples, now. The plane will pick you up exactly 24 hours after it drops you off. Oh, and one last thing,” he said as he put down the device he was tinkering with, started searching in some drawers and finally took out a weird-looking communication device. “We'll contact each other with this.”

Kohaku took the device, barely able to fight back her tears. She was so overwhelmed by the generosity of the man whose life she had almost taken... but now that she was thinking about it, a very obvious question was begging to be asked. 

"Mr. Ishigami..."

"Just Senku's fine."

"Senku... why are you helping me even though I was going to kill you?”

"You've got it all wrong," he smirked, as he went back to fiddling with the machine he had previously put down. "I'm not helping anyone. I just find scinetific challenges exhilerating." 

Kohaku felt a little offended at first but after a moment of epiphany, she chuckled to herself.

Senku claimed that he didn’t kill people because science could protect him, and that he would help people in the sake of practicing science, but Kohaku could tell that there was more to him than just his obsession with science. Yes, he definitely _was_ a bit of a mad scientist, but he was also much more than that. He was a genuinely kind person.

“Thank you, Senku,” she said before turning to head to the plane.

“What are you thanking me for, dummy?" he scoffed without raising his head from his workbench. "You're supposed to do that _after_ your sister recovers.” 

Kohaku smiled as she took a last glance at the busy scientist, then finally made her way to the jet, overcome with joy, and grateful that fate had allowed her to meet—and prevented her from blindly killing—a truly amazing individual. 


	3. Chapter 3

Senku and Kohaku's plan to find a cure for Ruri was set in motion and everything seemed to be going smoothly. Senku even allowed Kohaku to bring her and Ruri’s friend Chrome, a 5th year medical student who insisted on offering his help. After talking to him on the communication device he had given Kohaku, Senku acknowledged that the young man had a lot of potential and that his assistance could come in handy. And indeed, after only four days of hard work, the two scientists' joint efforts culminated in a breakthrough. They finally figured out the nature of Ruri’s illness and only needed two to three more days to develop a cure.

However, with Ibara in the picture, things were bound to take a turn for the worse sooner or later. What was especially alarming was that he seemed to be growing increasingly impatient with Kohaku, as she managed to conclude from his incessant phone calls, one of which she received as she was in the hospital with her sister.

“It has been a week, and I still haven't received Ishigami's head in a bag. When are you going to stop screwing around and finish your damn job?! If you can't do it, just give me my money back and—”

“I already told you I’m working on it! You don't have to keep calling me every day! No one knows where this guy lives and I’m no damn exception! I told you that I'll get the job done and I will, so don’t call me again or I’m coming for your head instead!” blurted Kohaku then, seething with rage, hung up without waiting for Ibara's reply.

It was becoming rather clear to her that he was the bad guy all along. If only Senku hadn't asked her not to kill him...

“Kohaku, is everything ok?” asked Ruri feebly, as her sister walked back into her hospital room. “Why were you shouting?”

“It’s nothing you should worry yourself about,” replied Kohaku, taking her infirm sister's hand into her own. “It’s just a problem at work… I’ll deal with it later.”

“Kohaku,” started Ruri, squeezing her sister's hand tightly, as her eyes brimmed with tears. “Please don’t push yourself because of me. I’ve been nothing but trouble for you, and dad, and Chrome. I just can’t live like this anymore...knowing that I'm causing you nothing but pai—”

“I told you to stop saying that kind of stuff!” snapped Kohaku. “Senku has found a cure! All you have to do is hang in there a bit longer, just for two days, and then everything's goi—”

Before Kohaku could finish her sentence, though, came the scene that she'd had to bear too many times for her own sanity. For years, she'd been forced to helplessly watch as her sister fell prey to incessant coughs that made it difficult for her to breathe. Whenever it would happen, Kohaku's mind would automatically think that that moment was going to be her sister's last. But thanks to Senku, her living nightmare was going to be over soon. She and Ruri only had to wait for two more days, and before they both even realized it, the two days had already gone by.

Kohaku finally received the fateful call from Senku, and immediately set her destination to the island. Shortly after she boarded the plane, however, her communication device rang again.

"Kohaku. You need to listen to me carefully. Whatever you do, make absolutely no visible reaction to what I'm about to tell you."

Kohaku could already tell what the situation was based on the information Senku had given her. She was in trouble and, if she wanted to make it to the island safely, she had to play her cards right.

"There is someone on the plane with you, but all you need to do for now is pretend that you're unaware of their existence. The logical thing for you both to do now is to avoid a fight aboard the jet, but the moment you land, they're most likely going to try to kill you and come for me next, but I need you to stall them for a bit. Chrome and I have a plan."

"Alright. I trust you'll cure Ruri no matter what. Thanks for all your help, Senku," she said at an attempt to trick the intruder into thinking that the subject of their conversation had nothing to do with them.

"Good luck, lioness," said Senku, clearly having understood Kohaku's intention. "Be careful."

The moment she set foot outside the plane, Kohaku climbed atop it with extreme agility and laid low in wait for the intruder to show up, but she was very soon surprised to hear a familiar clicking sound of a gun's safety behind her.

"Today's my lucky day," started a gruff voice coming from behind her. "Seems like I get to play with a beauty."

Kohaku rolled her eyes. Having been in the business of taking lives for a while, being held at gun point was child's play for her. With a swift turning motion, she kicked the gun out of her attacker's hand, then got her guns out and took a fighting stance, but the moment she laid eyes on her opponent, she was immediately forced to dodge as he had already pulled out a second gun and started shooting.

Trying to find cover, Kohaku could almost feel her heart beating out of her chest and it wasn't just because of the Adrenaline. She was beside herself with panic, because she knew that she was most likely not going to live to know whether her sister was cured or not. She recognized the person she was up against and knew that he was an unstoppable force of nature. He was the most renowned hitman in the world, and he went by the name of Mozu. Once he was hired to kill someone, it would be impossible for that person to avoid being murdered no matter how hard they tried. Wherever Mozu went, he was accompanied by death, and both of them were equally inescapable.

* * *

How long had it been since she had started exchanging gunfire with Mozu? Had she already bought Senku and Chrome the time they needed? It didn't matter. As long as she still had fight in her, it was best to keep stalling. She couldn't forgive herself if anything happened to Chrome and Senku. 

"You don't give up, do you?" said Mozu in amusement, from a distant location. "I guess you're as strong as they say you are, Miss Kohaku. It will be such a shame to kill you."

"The only one who's going to die today is you, Mozu," she said from behind her cover, fully aware that she was lying through her teeth.

It was a bit quiet for a few seconds after that, then the next thing Kohaku heard was an "I don't think so" to her left. Mozu snuck up on her in with the aim of taking the fight to a more close-range stage. When Kohaku jumped to dogde his knife attack, she fired her guns and, to her dismay, she had run out of bullets. She normally didn't make such a rookie mistakes as running out of Ammo, but Mozu was giving her no time to think. In any case, she wasn't at a disadvantage. Mozu must have ran out of bullets as well, or else he would have had no reason to attack her up close.

"Well, well. Seems like we're going to have to do this the old-fashioned way," he remarked.

"Fine by me," she replied defiantly, as she took out her knives and charged at him.

Unfortunately for her, she soon found herself outmached and a bloody mess. She was out of breath and barely able to stand, while Mozu was practically unscathed.

"I have to give it to you," he started condescendingly. "No one has ever been able to stand their ground against me for this long. You're something else, Miss Kohaku. I really don't want to see you go down. How about we stop the nonesense here, and grab dinner together once I'm done with the scientist?"

"Like I would ever go out with someone like you," she huffed, wiping some blood from her lips. "Don't be delusional."

And just as she was about to charge at him in one last potentially suicidal attack, an alarm went off followed by an announcement that all defense systems around Senku's house were going to be engaged in 20 seconds.

"I guess this is goodbye, Miss Kohaku. You've been a formidable opponent. I hope I'll get to see you again soon," he said before he vanished immediately afterwards.

Kohaku couldn't even see which direction he went due to her blurred vision, and just as she took a few steps towards the house, everything went black.


	4. Chapter 4

Kohaku came to in an unknown location. She was in someone's bedroom but she didn't know whose. As her vision stabilized, along came her last memories of what had happened. She hurried to leave the bed but a familiar voice stopped her.

"Woah there," said Chrome. "Take it easy. You shouldn't be getting up so fast. You've lost a lot of blood and you have a concussion, along with one broken rib, several bullet wounds, a dislocated shoulder, and countless other minor injuries."

"I don't care," she replied stoically. "Where's Mozu? Is Senku Ok?"

"You're asking about _me_ in that kind of state? That concussion must have really messed with your head," snickered Senku, as he leaned against the doorframe with his arms crossed. "You do realize that when I said stall the intruder, I didn't mean get yourself killed in the process, right?" he scolded.

"Where is Mozu?" she said, paying no attention to his patronizing words. "How are you two still alive?"

"Don't worry about him. He won't be a problem anymore. And as to how we're still alive... well, you're in for quite the story."

Senku then explained to Kohaku that everything that had happened—excepting her severe injuries—was all his plan from the start, including allowing Mozu on the island, and it was time for her to discover the reason why for herself.

"Can you walk?" asked Senku.

"Yeah," she replied.

"Then you two should join me," he said, leaving the room. "You lucky bastards are going to get a front-seat view of my latest scientific achievement."

As soon as she walked out of the bedroom, Kohaku couldn't believe her eyes when she saw Mozu passed out and tied up in Senku's living room, with his mouth covered in duct tape. After all the trouble she went through just to land a few hits, Senku had managed to capture Mozu without even so much as a single scrach on his body. She really was no match for his science.

She wanted to ask if the feeble-looking rope that he was tied in was enough to restrain him if he woke up, but she had no doubt that Senku knew what he was doing. 

"It might look fragile," started Seku, as though he had read Kohaku's mind. "But Graphene is 10 times stronger than steel with only 5% the density, so this guy isn't going anywhere. Isn't that right, hitman?" he said slapping his hand across Mozu's back and waking him up. "Time to get this show on the road," he continued. "Byaku-5, get the camera ready."

Then one of Senku's robots announced that they were going live, then started a countdown at the end of which Senku started his announcement.

"Citizens of earth, I, Senku Ishigami, would like to share with you my latest invention. Here we have the man who was sent to kill me, and he's going to be serving as my involuntary test subject, today." He then took out a syringe, the contents of which he injected into Mozu's arm as the man kept helplessly trying to squirm out of his ties.

"What I've injected into this man's veins is a truth serum. Yes, you heard me right. Not a laxative drug; a truth serum."

Kohaku turned to Chrome to communicate her amazement, but he just smiled confidently like it was no news to him. Apparently, aside from Mozu, she was the only other person who had no idea what was going on. 

"Now, I think that that should have been enough time for the serum to kick in," said Senku after having explained the science behind his serum to his audiences. "Time to give our subject a little test," he added, removing the duct tape from over Mozu's mouth.

Kohku was expecting him to be screaming and cursing, but Mozu was calm. He seemed to be in a trance-like state and only spoke when Senku addressed him.

"What's your name?"

"Mozu."

"What's your profession?"

"Gun-for-hire."

"How many targets have you successfuly eliminated?"

"124."

"How many targets have you failed to eliminate?"

"1."

"Who is that target?"

"Ishigami Senku."

Kohaku's eyes widened as she quickly realized what Senku was trying to achieve. All along, his big plan was to use Mozu as his ticket to finally getting rid of Ibara once and for all.

"And who hired you to kill Ishigami Senku?"

"Ken Ibara."

Ibara was in jail the following day, and a few days after that, Ruri was healed. Senku's truth serum—which he had concluded work on with Chrome's help during Kohaku and Mozu's fight—became the most recent talk of the media. Senku eventually shared his formula with the scientific community and not long after that, the serum found its useful applications in police investigations and courts of law. News of his development of a cure for a disease that the public barely had any awareness of, jointly with a previously unknown doctor in training, also flooded the media once Ruri was cured. 

* * *

A couple of days after her sister's recovery, Kohaku took Ruri and Chrome on a visit to Ishigami island. 

"Mr. Ishigami," said Ruri shaking Senku's hand. "It's an honour to finally meet you in person. I don't think there can ever be a way to repay you for your kindness towards my sister and myself." 

"Don't be mistaken. That drug wasn't made just for you. It was made in the name of science," he said bluntly. "You just happened to be the first beneficiary." 

Kohaku rolled her eyes, having predicted that Senku would say something of the sort. It was a good thing she warned her sister about how untactful he could be in social situations. 

"And for that I couldn't be more thankful," replied Ruri, with a warm smile on her face. 

"Hey Senku," started Kohaku, with a certain aim in mind. "Whatever happened to the engine modifications you said you were going to make to your spaceship?" 

"Oh it's too late for that now," he said in a slightly disheartened tone. "The bet was that I could finish it by tomorrow, but I only resumed work on the blueprints yesterday. There's no way I'm going to finish building it in one day."

"Oh... that's too bad," she said looking at Chrome and smiling. "Hey, close your eyes. Chrome and I wanna show you something." 

"How can you show me something if my eyes are closed?" he said sarcastically. 

"Oh for the love of God! You know what I meant," she sighed. "You know what? _I'll_ do it," she said walking behind him and covering his eyes. 

She and Chrome then led him to where his spaceship was and when Kohaku allowed him to open his eyes, she might have been mistaken but almsot looked like he was about to shed a tear.

"Heh heh heh. Chrome, you bastard, this was your doing wasn't it?" he said excitedly as he started examining his rocketship's new engine. 

"It was actually Kohaku's idea," replied Chrome. "And she helped me a lot too since your blueprints were pretty clear on what needed to be done. And the Byakus were pretty useful too, so yeah, it wasn't rocket science... Well, I mean literally speaking it _was_ , but you get what I'm trying to say," he chuckled.

Senku glanced at Kohaku with an actual smile on his face instead of a smirk for once, which sent a wave of warmth into Kohaku's heart. 

"Not so bad, you two," he finally said. "It's not quite there yet, but you definitely did most of the work. I think we can get it ready by tomorrow."

With a bit of help from Ruri, Kohaku, Chrome and his Byakus Senku did manage to finish the modifications to his engine in time and won the bet. But it turned out, as he had predicted, that his NASA colleagues still needed a few weeks, or possibly months, to finish working on their engine, so he was in no hurry to go to Mars yet. Instead he proposed to take Kohaku and her friends on a life-changing trip to outer space.

When they were back, it was time for to part ways, seemingly for good, but as Kohaku, Chrome and Ruri were boarding the jet, Senku decided he had something to say.

"Hey, Kohaku. You still have that communication device, right?"

"Yeah."

"Good. Give it to Chrome and Ruri. You're going to be staying on the island with me." 

Kohaku swallowed and felt a flush rise to her cheeks. 

"What?" 

"You once said that if I healed your sister, you would kill Ibara and everyone else who would dream of getting in my way, right?" 

"Y-yes...?" she replied in confusion.

"Yeah, that's called being a bodyguard, and it's not something you can do from a distance."

In reality, there was no reason for Senku to need a bodyguard, especially since he claimed that science was his preferred method of self-defense. He simply wanted Kohaku's company and realizing that almost gave her a heart attack. 

"I'm not going to force you to stay if you don't want to, of course. And in case you do choose to stay, you can feel free to leave the island and visit your family whenever you want," he added, seemingly indifferently.

"I would love to stay," she said with a smile that Senku reciprocated. 

* * *

After Ruri and Chrome left, the first days on the island with Senku started off with a lot of awkwardness that stemmed from both of them realizing they had feelings for each other that neither of them, being equally socially inept, knew what to do with, but they soon went back to their routine mutual derision and conversation went back to its natural flow. Neither of them took big steps, but they both dropped hints that they liked each other here and there, from Kohaku convincing him to watch shows with her and pretending to fall asleep on his shoulder, to him building her an entire training range. But one fateful night, as they were having dinner, everything changed.

"I'm going to Mars tomorrow," declared Senku suddenly. 

"Your NASA friends are almost done with their engine, I guess?”

“Yeah. They only still need a few more days.”

“When will you be back?" 

"The trip there should take around two months, and I'm going to need to spend a couple more there for research, so most likely a little less than a year from now."

"Oh... That's a long time," she said, visibly dejected that she wasn't going to get to see him for almost an entire year. 

"I want you to come with me," he announced, bringing her close to spitting out her food in shock.

"To Mars?!"

"Yes." 

Kohaku had almost never seen Senku so serious before. She only saw him like that when he was nervous and that in itself was a rare sight, so Kohaku found it rather amusing that the most confident man she'd even known was being suspiciously over-assertive. 

"Last time I checked, there were no living beings on Mars. Are you sure you're going to need a bodyguard there?" she joked.

"Company's more important than protection in outer space,” he stated factually, taking her off-guard. “So I’m not asking you come with me as my bodyguard."

“Oh, so you’re finally admitting that we’re friends?” she asked playfully.

“Incorrect,” he said passing a tiny velvet box towards her, and leaving the dinner table.

When Kohaku opened the box, tears came to her eyes but they quickly disappeared when she started laughing and shaking her head at the most anti-climactic, Senku-like marriage proposal in history. She put on the ring, then stealthily snuck up behind him.

“And if I said no?” she said, as she wrapped her arms around him and rested her head on his back.

“You’d be 10 billion percent lying,” he smirked. 

**Notes for the Chapter:**

> I have this headcanon that Senku would call his robots after his father: Byaku (short for Byakuya) and it sounds like a cool robot name!
> 
> Also, thank you for reading and leaving your comments and kudos!!! I hope you'v enjoyed this work ^^


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